If every picture tells a story, then so does every byelection. Don't fall for the official Labour blah that tries to belittle the result of this week's Henley contest to replace Boris Johnson as just another midterm byelection in an ultra-safe Conservative seat producing a predictable anti-government result. No, Henley was much more important than that.

It may be tempting, especially on the first anniversary of Gordon Brown's miserable year, to kick a man when he's down and concentrate on Labour's abject performance this week - fifth place behind the Greens and the BNP, a lost deposit, the collapse to 3% of the vote. Much of the instant coverage has already done just that. Actually, the real story from Henley - just as from Crewe and Nantwich a month ago - is rather less about Labour and rather more about the Conservatives.

What short memories some people have. Until four weeks ago, byelections were to the Tories what penalty shoot-outs still are to England footballers. Before Crewe, the Tories had never captured a Labour seat in a byelection since Mitcham and Morden in 1982. But before Thursday's 7.5% swing in Henley, the Tories had not performed so well in a byelection in one of their own seats since Knutsford in 1979 either. Nine weeks after Knutsford Margaret Thatcher was quoting St Francis of Assisi on the threshold of No 10. Crewe and Henley should be seen as a groundbreaking pair. Between them, the two results mark another historic Conservative recovery.

To understand the significance of Henley, just compare the result there this week with what happened in the previous byelection in a safe Tory seat in southern England only two years ago. In June 2006 the Conservatives had to defend the late Eric Forth's 13,000 majority in Bromley and Chislehurst. Just as at Henley, the Labour vote collapsed at the ensuing byelection - Labour finished fourth behind Ukip that time. But in Bromley large sections of the Tory vote deserted the cause too, bringing a surging Liberal Democrat challenger to within 633 votes of capturing the seat in one of those trademark byelection coups.

At Henley the Conservatives were defending a 12,000 majority from 2005, but the dynamic was absolutely different. Once again the Labour vote drained away. This time, though, the Tory share of the vote not only held up - as it had dramatically failed to do in Bromley - but actually strengthened. The Liberal Democrats deployed all their formidable byelection black arts to try to win Henley but ended up with a share of the vote only 2% greater than in 2005, compared with the 18% increase they notched in Bromley.

If you are looking for the real significance of Henley, this was it. For a quarter of a century and more, no byelection in a Conservative seat in southern England has been safe from massive tactical voting against the incumbent party, mainly to the benefit of the Lib Dems. Think Eastbourne, Newbury, Christchurch, Eastleigh and Winchester, famous captures all. The same very nearly happened in Bromley. But it didn't come close to happening in Henley. In Henley, decent and dissatisfied middle-ground voters voted Tory in serious numbers. The Lib Dem challenge was repulsed on a scale hardly seen for a generation.

This is David Cameron's doing. Henley does not tell us what sort of parliamentary majority Cameron will have on the morning after May 6 2010 - the hot favourite date of the next general election. But, all other things being equal, it points towards it being a healthy one. Add in the local election results and the latest national opinion polls, and it is hard to mistake the fact that the Tory leader has now led his party across an electoral watershed: a 20-point lead over Labour in the May local elections; another 20-point lead in the Guardian's latest poll this week; an 18-point lead in the Telegraph poll yesterday; two emphatic byelection wins - the Tory party has not had such depth of electoral success in opposition since the late 1960s under Ted Heath and the late 1970s under Thatcher. Two years is a long time to sustain such levels of success. But these trends will take some reversing now.

The blunting of the Liberal Democrat challenge has been a crucial part of this Conservative revival, as Henley proved. Senior Lib Dems will now tell you that their general election strategy for 2010 is to defend in the south against the Tories and to attack in the north against Labour. This sounds impressive, and it has historical plausibility - the aim to replace ailing Labour as the true progressive party of the future makes sense for Nick Clegg. It makes less sense, though, when you look at the electoral map, which reveals only 17 northern England Labour-held constituencies in the Lib Dem's 100 most winnable seats as against 48 southern England Tory-held ones. The truth is that Clegg's north-south strategy has been forced on him by the Tory revival.

Nevertheless, the crucial factor in Cameron's ascendancy has been Labour's collapse under Gordon Brown. One pities a man as proud as the prime minister studying this week's humiliating and embarrassing numbers. Just 2% of voters say their views of Brown have improved over the past 12 months (Guardian-ICM); a mere 3% think that he is an improvement on Tony Blair (Telegraph-YouGov). A year ago, when the pollsters asked if Brown was a liability to Labour, only 25% answered yes; today that figure has ballooned to 61%.

Right now, the talk among Labour ministers and MPs is overwhelmingly focused on day-to-day things. Who did better today - Brown or Cameron, Labour or the Tories? Was Friday better than Thursday and Thursday than Wednesday? Getting through to the summer recess without things getting any worse is now the sole focus of Labour's hugely deflated ambitions. Survival is all. After that, something may turn up.

Judged by this utterly uninspiring yardstick, this has not been Brown's worst week. Judged by almost any other criterion it is an abject and perhaps even an epochal collapse. While recognising the force of all the arguments against a change of Labour leader, it is hard to see how things can continue in this way for another two years.